1. Field of the Invention
The present invention relates to a method for purifying hexafluoroethane (hereinafter referred to as "FC-116" or "CF.sub.3 CF.sub.3 ") and, more particularly, to a method for purifying FC-116 containing, as impurities, hydrofluorocarbons including two carbon atoms in a molecule (hereinafter referred to as "HFCs"), thereby reducing the impurities of HFCs.
2. Related Background Art
FC-116 is used, for example, for dry etching of semiconductor and, therefore, high-purity FC-116 is demanded.
Various methods have been proposed heretofore as to methods for producing this FC-116.
Specific examples of the methods well known include (1) electrolytic fluorination using ethane and/or ethylene as a raw material, (2) thermal decomposition for thermally decomposing tetrafluoroethylene or the like, (3) methods for fluorinating acetylene, ethylene, and/or ethane by use of metal fluoride, (4) methods for fluorinating dichlorotetrafluoroethane or chloropentafluoroethane or the like by use of hydrogen fluoride, (5) direct fluorination for letting fluorine gas react with ethane or a hydrofluorocarbon, and so on.
However, when FC-116 is produced by these methods, FC-116 as an objective product forms an azeotropic mixture or an azeotrope-like mixture with intermediates or by-products produced in reactions to FC-116 or with the hydrofluorocarbon used as a raw material. Therefore, the problem is that separation of FC-116 is extremely difficult.
For solving this problem, an example proposed is a purification method of FC-116 for treating FC-116 containing the impurities of chlorotrifluoromethane (CClF.sub.3) and/or trifluoromethane (CHF.sub.3) including one carbon atom with an adsorbent such as active carbon or a zeolite (U.S. Pat. No. 5,523,499).
There existed, however, no easy, economical, industrially advantageous method for purifying FC-116 containing the impurities of HFCs including two carbon atoms in a molecule and thereby producing high-purity FC-116 little containing the HFCs.